A vane pump, which is one example of the abovementioned vane-type hydraulic device, is generally composed by installing vanes so as to be retractable in a radial direction, in each of a plurality of slots formed in a rotor, the rotor on which the vanes are installed being accommodated in an eccentric fashion with respect to an inner peripheral surface of a pump housing. In this way, by causing the rotor to rotate while the vanes press against the inner peripheral surface of the pump housing, with the rotor in an eccentric state with respect to the inner peripheral surface of the pump housing, it is possible to alter the volume of the pump chambers which are formed when the vanes demarcate the gap between the rotor and the pump housing, in accordance with the rotation of the rotor. The vane pump is composed in such a manner that fluid is taken in and discharged by altering the volume of the pump chamber in this way. In other words, fluid is taken into the pump chamber in the portion where the volume of the pump chamber increases, and conversely, the fluid in the pump chamber is discharged in the portion where the volume of the pump chamber decreases.
As described above, as a mechanism for forming a pump chamber by constantly pressing vanes against the inner peripheral surface of a pump housing which is provided eccentrically with respect to a rotor, a mechanism is known in the prior art, in which vanes are pressed against the inner peripheral surface of the pump housing with a force corresponding to the discharge pressure, by guiding the discharge pressure of the fluid to a vane back pressure chamber. Furthermore, a mechanism is also known in the prior art, in which vanes are pressed constantly against the inner peripheral surface of the pump housing, regardless of the amount of eccentricity of the rotor with respect to the inner peripheral surface of the pump housing, by using the impelling force of a spring to cause the vanes to project.